The upper limit of the signal processing frequency for processing electric signals in an integrated circuit is determined by active elements, such as transistors, and passive elements, such as transmission lines. Although the transistor cut-off frequency as of 2002 has reached 500 GHz, because the upper limit of the signal processing frequency remains about 100 GHz, the upper limit of the signal processing frequency in an integrated circuit is around 100 GHz. High-frequency signals are propagated in microstrip waveguides in packaged substrates, and coplanar waveguides in semiconductor substrates.
FIG. 5 shows a structural cross-section of a microstrip line. The microstrip line comprises a signal electrode 101, a semiconductor substrate 102 and a ground electrode 103. Microstrip lines have the merit of being easy to construct and cheap to manufacture. However, although the propagation of an electric signal in a microstrip line takes place mostly in the semiconductor substrate 102, a portion of the electric signal exists in air on the signal electrode 101 side. The resulting difference in the phase velocity of the signal in the dielectric and in the air gives rise to a hybrid mode having electric and magnetic fields in the direction of propagation. In high-frequency wave regions, a hybrid mode is a predominant cause of radiation loss. Moreover, as the signal frequency increases, the signal wavelength decreases, manifesting the hybrid mode. Therefore, microstrip lines are not suitable for high-frequency transmission, and are mostly applied to packaged substrates where the frequencies are relatively low.
Coplanar waveguides will now be explained. FIG. 6 shows the cross-section of a coplanar waveguide. This coplanar waveguide comprises a signal electrode 105 and ground electrodes 106 disposed on a semiconductor substrate 104, with the signal electrode 105 arranged between the ground electrodes 106. An advantage of this configuration is that it is easy to mount semiconductor devices. Since the ground electrodes 106 are adjacent to the signal electrode 105, the electric signals are localized between the signal electrode 105 and the ground electrodes 106. Because of the spatial localization of the electric signals, a coplanar waveguide can cope with higher frequencies than a microstrip line. However, as in the case of a microstrip line, the signal is propagated in the semiconductor substrate 104 and in the air, giving rise to mismatched phase velocities. Consequently, a drawback is that as the frequency rises, the hybrid mode becomes manifest, increasing the radiation loss.
Next, a transmission line will be described that can transmit signals at higher frequencies than a microstrip line or coplanar waveguide. As of 2002, the line that can realize the highest signal transmission frequencies is said to be the nonlinear transmission lines (NLTLs) proposed by M. J. W. Rodwell. This is a type of active transmission line in which a capacitor is located between the signal and ground lines. An NLTL utilizes the non-linear nature of the capacitance relative to the voltage to compress electric pulses and enable transmission of high-frequency signals.
In the case of an NLTL, compression of the electric pulses has so far been achieved by utilizing the capacitance of Schottky junctions between metal and semiconductor (M. J. W. Rodwell et al., Proc. IEEE, vol. 82, No. 7, pp 1037-1059(1994)). Also, by utilizing this active line, a prototype sampling circuit was fabricated that was capable of sampling signals at 725 GHz (U. Bhattacharya, S. T. Allen and M. J. W. Rodwell, IEEE, vol. 5, No. 2 (1995)).
FIG. 7 shows cross-sectional and plan views of this NLTL. Looking at the cross-sectional view of FIG. 7(a), it has a coplanar structure with a signal line on a semiconductor substrate 107 and ground lines at each side. The signal electrode 108 that constitutes the metal electrode of the signal line, and the ground electrodes 109 that constitutes metal electrode of the ground lines, contact N-type conductive semiconductor 110a and 110b. The ground electrodes 109 are in mutual contact via the N-type semiconductor 111. The N-type semiconductor 110a below the signal electrode 108 is insulated from the ground lines 109 and from the N-type semiconductor 111 by insulation layer 112. The ground lines 109 are each comprised of a main line 109a and a plurality of extended portions 109b that extend in the direction of the signal line 108 and form a Schottky junction (denoted by “SJ” in the drawing) with the N-type semiconductor 110a provided below the signal electrode 108.
In the NLTL thus configured, the extended portions 109b of the ground electrodes 109 generate an added inductance component. Obtaining a balance between the nonlinear capacitance produced by the Schottky junction and the added inductance component has a major effect on the electric pulse compression characteristics.
However, a signal propagated by an NLTL propagates in the semiconductor substrate and the air over the transmission line, so that, like in the case of a coplanar waveguide, an NLTL is an inhomogeneous waveguide. The difference in the phase velocity of the signal in the semiconductor substrate and in the air gives rise to a hybrid mode and generates electric and magnetic field components in the direction of propagation. In high-frequency wave regions, radiation loss caused by hybrid modes cannot be ignored.
In NLTLs, moreover, with respect to the signal electrodes, the nonlinear capacitance becomes a problem because it exists in the regions between the signal electrode and the ground electrodes. Since the energy of the electric signals propagating in an NLTL exists between the signal and ground electrodes, the use of numerous metal electrodes for Schottky junctions in that region produces impedance mismatches, producing reflection and scattering of traveling electromagnetic waves. With reference to the plane view of FIG. 7(b), for example, electromagnetic waves having a wavelength that is not greater than the gap between the electrodes forming the Schottky junction where there is an impedance mismatch suffer a major increase in loss due to multiple reflections.
In addition, the metal electrodes forming the Schottky junction have a boundary condition effect on propagating electric signals, complicating the signals and making it impossible to represent the transmission mode as analogous to TEM mode. There are also problems arising in the semiconductor manufacturing process due to the utilization of compound semiconductor Schottky barriers in NLTLs. The capacitance of a Schottky junction is determined by the height and thickness of the Schottky barrier. The thickness of a Schottky barrier is inversely proportional to the defect density at the compound semiconductor interface, and this defect density is highly dependent on the surface treatment method used: in current semiconductor manufacturing processes, uniform surface treatment cannot be reproduced. Thus, the difficulty in reproducing the size and uniformity of the nonlinear capacitance that is the critical parameter of an NLTL is a major problem in terms of practical utility.
In view of the above, an object of the present invention is to provide an electric signal transmission line that is capable of high-speed electric signal propagation and can be manufactured with stable quality using current semiconductor manufacturing processes.